1. Field of the Invention
The present invention relates to a resolution correction apparatus and program for correcting resolution of an image captured by a camera for use in a portable device, an electronic still camera, or the like, and to a computer-readable recording medium having the same program recorded thereon.
2. Description of the Related Art
Cameras for use in small-sized portable devices, electronic still cameras, etc., are becoming smaller in size. A lens provided in such a camera has a lens aberration unique thereto and is characterized in that resolution of an image captured by a camera including such a lens becomes lower at peripheral portions of the image than at a central portion thereof. The image captured by the camera having such a lens is unclear since the resolution of the image is lower at the peripheral portions of the image than at its central portion corresponding to the lens center.
When the number of lenses are increased in order to prevent resolution from being degraded at the peripheral portions of the image, a size of a lens, and weight and cost of a camera employing the lenses are increased. There are great demands for reduction in production costs and size of an optical system such as a lens, and thus it is not easy to increase the number of lenses used in a camera, such as a camera for use in a portable device, an electronic still camera, or the like.
Japanese Laid-Open Patent Publication No. 7-74987 discloses a structure for preventing a reduction in resolution at the peripheral portions of an image without increasing the number of lenses. FIG. 10 is a block diagram of a conventional resolution correction apparatus 9 disclosed in this publication. The resolution correction apparatus 9 has an input terminal T to which an image signal representing an image captured by a camera is input. The image signal input to the input terminal T is supplied to each of a correction addition section 91, an outline correction signal generation section 89, and a correction amount adjustment section 93. The outline correction signal generation section 89 has a horizontal correction signal generator 90. The horizontal correction signal generator 90 generates a horizontal outline correction signal based on the image signal input to the input terminal T and outputs the generated horizontal outline signal to a horizontal variable amplifier 94 provided in the correction amount adjustment section 93.
The correction amount adjustment section 93 also includes a horizontal parabolic wave generator 95 which receives the image signal input to the input terminal T and outputs a horizontal parabolic wave signal in synchronization with the image signal to the horizontal variable amplifier 94. The horizontal variable amplifier 94 nonlinearly adjust the horizontal outline correction signal generated by the horizontal correction signal generator 90 based on the horizontal parabolic wave signal generated by the horizontal parabolic wave generator 95, such that correction amount becomes great at the left and right edge portions of the image represented by the image signal.
An output of the horizontal variable amplifier 94 is supplied to a horizontal adder 92 provided in the correction addition section 91. The horizontal adder 92 delays the image signal input to the input terminal T so as to be in timing with the horizontal outline correction signal adjusted by the horizontal variable amplifier 94 and adds the delayed image signal to the horizontal outline correction signal. This addition emphasizes the horizontal outline of the image represented by the image signal.
An output of the horizontal adder 92 is supplied to a vertical correction signal generator 96 provided in the outline correction signal generation section 89 and a vertical adder 97 provided in the correction addition section 91. The vertical correction signal generator 96 generates a vertical outline correction signal based on the image signal representing the image having horizontal outline emphasized by the horizontal adder 92.
The correction amount adjustment section 93 also includes a vertical parabolic wave generator 99 and a vertical variable amplifier 98. The vertical parabolic wave generator 99 outputs a vertical parabolic wave signal in synchronization with the image signal input to the input terminal T to the vertical variable amplifier 98. The vertical variable amplifier 98 nonlinearly adjusts the vertical outline correction signal generated by the vertical correction signal generator 96 based on the vertical parabolic wave signal output by the vertical parabolic wave generator 99, such that the correction amount becomes great at the upper and lower edge portions of the image represented by the image signal.
An output of the vertical variable amplifier 98 is supplied to a vertical adder 97 provided in the correction addition section 91. The vertical adder 97 delays the image signal representing the image having the horizontal outline emphasized by the horizontal adder 92 so as to be in timing with the vertical outline correction signal adjusted by the vertical variable amplifier 98 and adds the delayed image signal to the vertical outline correction signal. This addition further emphasizes the vertical outline of the image having the emphasized horizontal outline.
The image signal representing the image having the vertical outline emphasized by the vertical adder 97 is provided as an outline-corrected image signal to a circuit in a subsequent stage (not shown). In the image represented by the image signal in which the vertical outline is emphasized by the vertical adder 97, the horizontal outline is also emphasized, so that the outlines in the image are nonlinearly corrected so as to be greatly emphasized at the peripheral portions where resolution of a lens is low.
Resolution of an image captured by a camera becomes lower along directions from the center of a lens provided in the camera to the peripheral portions of the lens. For example, in the smallest concentric circle having the same center as that of the image, resolution is substantially at the same level in the entire area defined by the smallest concentric circle. This can be said of other larger concentric circles. However, in the larger concentric circles, resolution is substantially at the same level only in an area excluding the area defined by a smaller concentric circle(s). For example, in the second smallest concentric circle having the same center as that of the image, resolution is substantially at the same level in the entire area excluding the area defined by the smallest concentric circle. Therefore, in order to perform correction so as to achieve uniform resolution across the image, the amount of correction for the respective areas defined by the concentric circles having the same center as that of the image is required to be equal.
However, in the aforementioned conventional resolution correction apparatus 9, the vertical outline of the image is emphasized by the vertical variable amplifier 98 after the horizontal outline of the image is emphasized by the horizontal variable amplifier 94, and therefore the amount of correction is not equal in a concentric circle having the same center as that of the image. Therefore, there is a problem in that uniform resolution is not achieved across the image and the corrected image appears to be unnatural.